js_runtime: Split _asyncHelper into separate functions

sdk/lib/_internal/js_runtime/lib/async_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/async_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/async_patch.dart b/sdk/lib/_internal/js_runtime/lib/async_patch.dart
index 893a2ebb044f27f7f7de91f19bafdfc1be420b97..cd590cd31d57644f8626901d14a72684dd1c6bd1 100644
--- a/sdk/lib/_internal/js_runtime/lib/async_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/async_patch.dart
@@ -130,40 +130,41 @@ class Timer {
   }
 }
 
-/// Runtime support for async-await transformation.
-///
-/// This function is called by a transformed function on each await and return
-/// in the untransformed function, and before starting.
-///
-/// If [object] is not a future it will be wrapped in a `new Future.value`.
-///
-/// If [asyncBody] is [async_error_codes.SUCCESS]/[async_error_codes.ERROR] it
-/// indicates a return or throw from the async function, and
-/// complete/completeError is called on [completer] with [object].
-///
-/// Otherwise [asyncBody] is set up to be called when the future is completed
-/// with a code [async_error_codes.SUCCESS]/[async_error_codes.ERROR] depending
-/// on the success of the future.
-///
-/// Returns the future of the completer for convenience of the first call.
-dynamic _asyncHelper(
-    dynamic object,
-    dynamic /* int | _WrappedAsyncBody */ bodyFunctionOrErrorCode,
-    Completer completer) {
-  if (identical(bodyFunctionOrErrorCode, async_error_codes.SUCCESS)) {
-    completer.complete(object);
-    return;
-  } else if (identical(bodyFunctionOrErrorCode, async_error_codes.ERROR)) {
-    // The error is a js-error.
-    completer.completeError(
-        unwrapException(object), getTraceFromException(object));
-    return;
-  }
-
-  _awaitOnObject(object, bodyFunctionOrErrorCode);
+/// Runtime support for the async-await transformation. Initiates the

[floitsch, 2017/05/03 10:14:25]

The first sentence should say what the function does. It should also be a simple
sentence (and not a three line paragraph).

See below for a suggestion on how to format the doc.

[sra1, 2017/05/03 17:12:20]

Done.

+/// asynchronous computation of [bodyFunction].  Returns the future of the
+/// completer for convenience of the transformed code.
+dynamic _asyncStart(_WrappedAsyncBody bodyFunction, Completer completer) {
+  // Expansion and specialization of `await null`.
+  Function thenCallback =
+      (result) => bodyFunction(async_error_codes.SUCCESS, result);
+  // We can skip the zone registration, since the bodyFunction is already
+  // registered (see [_wrapJsFunctionForAsync]).
+  Future._nullFuture._thenNoZoneRegistration(thenCallback, null);
   return completer.future;
 }
 
+/// Runtime support for 'await' on [object] in the async-await transformation.
+/// Sets up [bodyFunction] to be called when the future is completed with a code
+/// [async_error_codes.SUCCESS]/[async_error_codes.ERROR] depending on the
+/// success of the future.
+dynamic _asyncAwait(dynamic object, _WrappedAsyncBody bodyFunction) {
+  _awaitOnObject(object, bodyFunction);
+}
+
+/// Runtime support for the async-await transformation. Returns [object] from

[floitsch, 2017/05/03 10:14:25]

Starting a sentence with "Returns" in a dartdoc puts me in "this function
returns" mode.

Maybe:

"""
Completes the future of the `async` function.

Used as part of the runtime support for the async/await transformation.

This function is used when the `async` function returns (explicitly or
implicitly).
"""

[sra1, 2017/05/03 17:12:20]

Done.

+/// the async function.
+dynamic _asyncReturn(dynamic object, Completer completer) {
+  completer.complete(object);
+}
+
+/// Runtime support for the async-await transformation. Throws from the async
+/// function.  [object] is a JavaScript Error object.
+dynamic _asyncRethrow(dynamic object, Completer completer) {
+  // The error is a js-error.
+  completer.completeError(
+      unwrapException(object), getTraceFromException(object));
+}
+
 /// Awaits on the given [object].
 ///
 /// If the [object] is a Future, registers on it, otherwise wraps it into a